When fire fighters, first responders, military personnel or construction workers overheat in hot, humid climates, they not only lose effectiveness and risk heat exhaustion and heat stroke, but can suffer significant cognitive impairment. While workers can remain safe by taking frequent rest breaks, time pressure to complete work frequently results in employees skipping rests, which can lead to heat related illness, some of which can be life threatening. For example, a 70-kilogram (154 lb), physically fit individual engaged in very heavy work (such as digging with a shovel) generates about 2000 Btu/hr (586 W) of heat. The heat capacity of tissue is about 3.5 kJ/kg° C. (0.836 Btu/lb° F.) and in the absence of any heat losses, absorbing this much heat will raise the body's core temperature from 37° C. (98.6° F.) to 40° C. (104° F.) in about 20 min. When the body's temperature reaches 104° F., the result is heat stroke, which is a medical emergency. At this core temperature, the body loses its ability to regulate temperature, and cannot cool itself down. If untreated, heat stroke can lead to permanent disability or death. Less severe but still serious, heat exhaustion occurs at somewhat lower core body temperatures (i.e. the body's thermoregulatory system is still functioning). Symptoms include: heavy sweating, paleness, muscle cramps, fatigue, weakness, dizziness, headache, nausea and sometimes brief loss of consciousness (fainting). While less severe than heat exhaustion, even mild overheating can cause painful muscle cramps and heat rash.
Sweating cools the body in all but the very hottest of dry climates (Sawka, M. N and Pandolf, K. B. (2002) “Physical Exercise in Hot Climates: Physiology, Performance, and Biomedical Issues,” Chapter 3 “Physical Exercise in Hot Climates: Physiology, Performance, and Biomedical Issues,” in Medical Aspects of Harsh Environments, Volume 1, Textbooks of Military Medicine, Office of the Surgeon General, Department of the Army, USA; D. E. Lounsbury; R. F. Bellamy and R. Zajtchuk (editors). However, in hot, humid climates, the cooling effect of sweating is generally not effective because of the body's limited surface area and because the rates of mass transfer from the liquid phase (sweat) to vapor phase are low (because high ambient humidity lowers the driving force for the evaporation of water). While a person can be cooled in humid climates using air conditioning, ice, circulating liquid cooling systems, cold packs and phase change materials, these methods are frequently impractical because of their weight and size. For ice, cold packs and phase change materials, a refrigerator or freezer is needed to regenerate the materials, which may be unavailable. Further, these devices are not self-regulating and therefore do not respond to changes in the metabolic heat load; they keep cooling even when the worker is at rest, causing overcooling.
Three methods for personal cooling are: 1) ice/phase change packs, 2) liquid cooling systems and 3) forced air cooling. Ice packs and phase change materials are held in pouches or pockets sewn into a vest and heat is removed as the ice or phase change material melts. Ice is better than phase change materials because it has a higher heat of fusion (ΔHf water=143 Btu/lb) and can therefore remove more heat per unit weight. While ice packs are simple, they have several disadvantages that make them impractical for mobile personal cooling; they are heavy (about 1000 g/L), overcool the wearer initially and undercool later, are dead weight after they melt, and need a freezer for regeneration. Liquid cooling systems are complex, heavy and can be hazardous in some situations. They require a pump (and large, heavy batteries to operate it), a heavy liquid reservoir and an electronic feedback system (vulnerable to failure) for temperature control. Natural (sweat) cooling uses air to evaporate sweat; the heat of vaporization of water is about 1000 Btu/lb, which is 7 times greater than the heat of fusion for melting ice. The problem is that in humid climates, sweat usually cannot evaporate fast enough to provide adequate cooling unless there is significant air movement.
Clothing and protective equipment can severely restrict the flow of air, and this interferes with the evaporative cooling effects of sweating. This is bad enough in a hot, dry climate, but in hot, humid environments, this can be extremely uncomfortable (which is a distraction) and potentially dangerous (if it results in heat exhaustion or heat stroke). Heat stress when wearing clothing and protective gear and the effectiveness of existing personal cooling technologies (frequently referred to as microclimate technologies) has been studied to determine both the physiological and psychological effects of overheating on the ability of people to perform a wide variety of tasks that require different levels of exertion (Cadarette, B. S.; Cheuvront, S. N.; Kolka, M. A.; Stephenson, L. A.; Montain, S. J. and Sawka, M. N. (2006) “Intermittent Microclimate Cooling During Exercise Heat Stress in U.S. Army Chemical Protective Clothing,” Ergonomics, Vol. 49, No. 2, 209-219). The amount of metabolic heat generated depends on the task that the worker is performing. Effort is therefore broadly divided into four categories: 1) very light work generates 105-175 W; 2) light work generates 175-325 W; 3) moderately heavy work generates 325-500 W; and 4) extremely hard work can produce more than 600 W of heat that must be removed. Especially for moderate to heavy work, frequent rests are required unless there is an active microclimate cooling technology being used by the worker. As expected, the harder the work, and the hotter and more humid the environment, the less time that can be spent working.
While ambient temperature and relative humidity provide guidelines for working in hot, humid climates are available, they do not account for radiative heating when a person is working in the sun. Therefore, formulas that use weighted combinations of dry bulb temperature, wet bulb temperature and in some cases the black globe temperature have been developed to calculate various heat indices (Epstein, Y. and Moran, D. S. (2006) “Thermal Comfort and the Heat Stress Indices,” Ind. Health, 44, 388-398). These indices are then correlated with experiments done with human volunteers to determine the weighting factors for each type of temperature. The basic idea is that an index provides a single number that fully describes the work environment. Human physiology and the psychological perception of heat and humidity are very complex, so a heat index provides a rough guideline. One commonly used index is the Wet Bulb Globe Temperature (WBGT) Index. It is calculated using the wet bulb temperature (Tnwb), the black globe temperature (Tbg), and the dry bulb temperature (Tdb). The dry bulb temperature is the temperature measured outdoors away from direct sunlight. The wet bulb temperature is the temperature measured using a sling psychrometer, and the black globe temperature is the temperature of the air contained in a black sphere that absorbs most of the solar radiation. The WBGT is calculated from WBGT=0.7Tnwb+0.2Tbg+0.1Tdb when outdoors in the sun, and WBGT=0.7Tnwb+0.3Tbg when indoors or in the shade. For low values of WBGT (in the twenties ° C.) heavy work can be continuously done without danger of overheating, but as the WBGT increases, progressively less time can be spent working safely without periodic resting. FIG. 6 shows recommended work/rest cycles and required water intake as a function of WBGT.
The only safe way to extend the time for heavy work when the WBGT is high is to have some way to cool the individual. Indoors, this might be as simple as using an electric fan or air conditioning. The problem is that all of the portable systems that one might consider for outdoor use fall short in one or more respects. There are three ways to actively remove metabolic heat from the body using portable systems: 1) ice/cold packs and other phase change materials, 2) liquid cooling and 3) air cooling. Each method has advantages and disadvantages. The advantage of ice/cold packs and phase change materials is that they are simple—the person wears a vest that contains pockets that hold the packs. Unfortunately, the disadvantages of ice/cold packs and other phase change materials outweigh their advantages and include: 1) the need for an external refrigerator/freezer to regenerate the packs, 2) the packs are deadweight once they are spent/thawed, but still need to be carried if they are going to be reused, and 3) there is no temperature control so they cannot be turned off when resting or when the work load is reduced. This is a serious problem since any system that has enough capacity to remove the heat generated during heavy work has more than enough capacity to dangerously overcool the user.
Liquid cooling systems circulate water through tubes next to the skin and their main advantage is that they have high heat transfer rates. Unfortunately, liquid systems require a refrigeration/chiller system to reject the heat from the warmed water and these are heavy, consume large amounts of power (thus requiring heavy batteries in a portable garment), and unless they use a complicated feedback temperature control system, they can overcool the user. In addition, a vest that uses liquid filled tubes (usually water) is heavy (1 kg/liter for just the water). Thus, a liquid cooling system means one has to carry a small refrigeration unit, which in addition to already being heavy and complex, has a very low efficiency (refrigeration system efficiency increases with increasing size).
Sophia D'Angelo, & William Lauwers, Apr. 30, 2009 (“The Cooling Vest-Evaporative Cooling”, a Major Qualifying Project Report in fulfillment of a Bachelor of Science Degree, Worchester Polytechnic Institute, Chemical Engineering and Mechanical Engineering, advised by Anthony Dixon) teaches a cooling vest containing an inner wicking layer, a middle mesh liner, and an outer shell. The vest contains a thermoelectric cooler and a powered fan to blow air over the thermoelectric cooler, providing air that is cooler than ambient air, which is then forced through the vest in the space between the wicking layer and the outer shell layer (See page 42). It further teaches a vest containing “thick Styrofoam” (page 42) structural supports that separate the wicking layer from the outer shell layer and that form channels to direct the airflow direction starting near the waist and flowing up and out the arm holes. Heat exchangers are added to improve the transfer of heat from the hot and cold sides of the thermoelectric cooling device (see pages 44-47). D'Angelo & Lauwers (2009) is incorporated by reference herein.
These references contain at least one of the following limitations: there are no small channels to control the rate of mass transfer of evaporated sweat, the garment is heavy, the garment requires consumables such as ice, the garment can over cool the wearer, or the garment is ineffective in hot and humid environments.
Thus, there is a need for a light, portable cooling technology that can be used by first responders, construction workers, fire fighters, military personnel and others to prevent heat exhaustion or heat stroke when working in hot or humid environments.